Aizen forces Orihime and Grimmjow to have sex in front of him. He wants them to create a baby between a human and arrancar. Eventually Aizen's plan is revealed.
The story was up on deviantART before it got removed and if I remember correctly it was a one-shot though I'm not very sure. I'd appreciate it if anyone could help me out, 'cause I really want to read it again.